Nitronyl nitroxides have been used to detect nitric oxide released from nitrosovasodilators. The vasorelaxant activity of Angeli's salt (Na2N2O3) has been attributed to formation of nitroxyl (HNO) species, followed by one-electron oxidation of HNO to nitric oxide. The objective of this study was to determine the effect of nitronyl nitroxide on the vasodilatory effect of Angeli's salt in myocardium. The reaction between nitronyl nitroxide and Angeli's salt was followed by ESR spectral changes from a five-line pattern from nitronyl nitroxides to a nine-line pattern for imino nitroxides. In isolated rat hearts (n=8/group), Angeli's salt (15 fmol/l) increased coronary flow to a maximum of 187q4% baseline, which was antagonized by hemoglobin (4 fmol/l) and nitronyl nitroxide (400 fmol/l) from 187q4% to 152q1% and 130q1% of baseline, respectively (p<0.05 as compared with control). We conclude that coronary vascular relaxation elicited by Angeli's salt is mediated by nitric oxide and that nitronyl nitroxide inhibits the vasorelaxation by trapping nitric oxide.